1. Field of the Invention
This invention relates to systems for monitoring electrical energy in electrical distribution systems, and more particularly to systems for improving the accuracy of measurement of electrical parameters by intelligent electronic devices.
2. Description of the Related Art
Monitoring of electrical energy by consumers and providers of electric power is a fundamental function within any electric power distribution system. Electrical energy may be monitored for purposes of usage, equipment performance and power quality. Electrical parameters that may be monitored include volts, amps, watts, vars, power factor, harmonics, kilowatt hours, kilovar hours and other power related measurement parameters. Typically, measurement of the voltage and current at a location within the electric power distribution system may be used to determine the electrical parameters at that location.
The voltage and current may be detected directly or using a transformer such as a current transformer or a potential (voltage) transformer. Transformers are typically used where the voltage and/or current are outside the acceptable range of devices used to monitor the electrical energy. Transformation of the magnitude of the voltage or current by transformers may be represented by a ratio. The ratio represents the difference between the voltage or current of the detected electrical energy and the corresponding voltage or current output from the transformer.
Transformers may be classified according to accuracy. Classification provides a comparative indication of the accuracy of transformation of a given transformer. An example accuracy classification system is provided by the ANSI/IEEE C57.13-1978 standard. In the ANSI/IEE C57.13 standard, the accuracy classes are established based on the percentage of transformation error a transformer exhibits at a particular voltage and/or current, frequency and burden. The transformation error is the difference between the design ratio and the actual ratio under operating conditions. The burden is the amount of electrical load connected to the output of the transformer and may be expressed as volt-amperes (VA) and power factor, or as total ohms impedance with an effective resistance and reactive component.
A known problem with existing systems of accuracy classification is the relatively large differences in the percentage of transformation error that may be acceptable within a given accuracy classification. In addition, some existing systems of accuracy classification use a predetermined set of testing parameters that may not represent actual operating conditions. Further, accuracy of the transformation of the voltage and current may vary as system conditions vary. Inaccuracy in the transformation creates inaccuracies in the electrical parameters derived from the transformed voltages and currents.
Where the electrical parameters are used, for example, for measuring energy usage by a device or facility, the inaccuracy may result in erroneous billing. Further, consumers of energy that are interested in the quality of the energy supply may be provided flawed data. In addition, in instances where energy usage is controlled based on current system conditions, inaccuracy of the amount of energy being consumed may result in erroneous control decisions. Accordingly, a need exists for systems capable of providing improved monitoring accuracy to provide precise measurement and reporting of electrical parameters.
The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. By way of introduction, the preferred embodiments described below include a system for improving the accuracy of monitoring electrical energy using metering sensors.
An intelligent electronic device (IED) is coupled with the metering sensors. The metering sensors may be transformers, or some other form of sensors capable of sensing electrical energy and providing an output. The output represents the sensed electrical energy transformed to electric signals compatible with the IED. The IED uses the output to calculate, monitor, store and display various electrical parameters. To improve the accuracy of the IED, characterization curves may be generated for the metering sensors. The characterization curves may be generated by testing the metering sensors under simulated operating conditions. The characterization curves may be empirically developed for a particular one of the metering sensors or for a predetermined group of metering sensors. The IED may selectively apply the characteristic curves during derivation of the electrical parameters to improve accuracy.
One embodiment describes a method of accurately monitoring electrical energy with an intelligent electronic device and a metering sensor. The method comprises testing the metering sensor in varying operating conditions and empirically developing a plurality of characteristic curves as a function of the varying operating conditions that affect accuracy. The method further comprises monitoring the varying operating conditions with an intelligent electronic device coupled with an output of the metering sensor and selectively applying the characteristic curves to the output as a function of the varying operating conditions.
Another embodiment describes a method of improving accuracy of an intelligent electronic device that monitors electrical energy with metering sensors. The method comprises testing a metering sensor to determine a characteristic curve, storing the characteristic curve in the metering sensor and accessing the characteristic curve with the intelligent electronic device coupled with an output of the metering sensor. The method further comprises applying the characteristic curve to the output of the metering sensor with the intelligent electronic device.
Yet another embodiment describes a method of self-testing to improve accuracy of an intelligent electronic device that monitors electrical energy. The method comprises testing a first metering sensor to develop a first characteristic curve. The method further comprises monitoring the electrical energy with the intelligent electronic device using the first metering sensor and a second metering sensor and applying the first characteristic curve to the first metering sensor to improve monitoring accuracy. In addition, the method comprises comparing monitoring performed with the first metering sensor and monitoring performed with the second metering sensor. Further, the method comprises generating a second characteristic curve for the second metering sensor with the intelligent electronic device. The second characteristic curve is generated as a function of differences between monitoring performed with the first metering sensor and monitoring performed with the second metering sensor.
A method of accuracy monitoring electrical energy with metering sensors and an intelligent electronic device is described by another embodiment. The method comprises empirically developing a characteristic curve for a predetermined group of metering sensors and selecting the characteristic curve for use during operation of the intelligent electronic device. The characteristic curve is selected as a function of a metering sensor coupled with the intelligent electronic device.
The method described by another embodiment involves improving the accuracy of metering sensors and an intelligent electronic device that monitor electrical energy. The method comprises empirically developing a characteristic curve for a metering sensor and storing the characteristic curve in a database. The characteristic curve is stored based on an identifier associated with the metering sensor. The method further comprised transferring the characteristic curve over a network to the intelligent electronic device as a function of the identifier of the metering sensor coupled with the intelligent electronic device.
An intelligent electronic device for monitoring electrical energy is disclosed by another embodiment. The intelligent electronic device comprises a metering sensor operable to measure electrical energy and provide an output during varying operating conditions. The accuracy of the output of the metering sensor is a function of the varying operating conditions. The intelligent electronic device further comprises a central processing unit coupled with the metering sensor. The central processing unit selectively applies at least one characteristic curve to the output as a function of the varying operating conditions. The characteristic curve is developed through testing to improve the accuracy of electrical parameters derived by the intelligent electronic device under the varying operating conditions.
Another embodiment discloses an intelligent electronic device capable of self-testing to improve the accuracy of monitoring of electrical energy. The intelligent electronic device comprises a first metering sensor and a central processing unit coupled with the first metering sensor. The central processing unit applies a predetermined first characteristic curve to monitoring performed with the first metering sensor. The intelligent electronic device further comprises a second metering sensor coupled with the central processing unit. The central processing unit compares monitoring performed with the first metering sensor to monitoring performed with the second metering sensor and generates a second characteristic curve for the second metering sensor.
Yet another embodiment discloses an intelligent electronic device for monitoring electrical energy. The intelligent electronic device comprises a central processing unit, a first metering sensor and a second metering sensor. The first metering sensor is coupled with the central processing unit. The second metering sensor is also coupled with the central processing unit. The central processing unit switches between the first metering sensor and the second metering sensor during monitoring.
A network distribution system for distributing characteristic curves for metering sensors is disclosed by another embodiment. The network distribution system comprises a network, an intelligent electronic device communicatively coupled with the network and a metering sensor coupled with the intelligent electronic device. The network distribution system further comprises a server communicatively coupled with the network. The server comprises the characteristic curves and may supply a particular characteristic curve to the intelligent electronic device.
Further aspects and advantages of the invention are discussed below in conjunction with the preferred embodiments.